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7075 aluminum

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JPSAXNC:
I've built two small compressed air motors for a model plane. The crank shafts are made in two pieces, on the first motor the front and rear crank shafts were made of 6061 alum. and rode in brass bushings. I gave all the moving parts a small drop of 30 weight oil, this motor worked well. On the second motor the front half of the crank was made from 7075 alum. riding in a brass bushing, the rear half of the crank was made from 304 SS. riding in a 7075 alum. bushing, all the moving parts got a small drop of 30 weight oil. I ran the motor for about one minute it seemed to work ok, I put it aside for a while and later when I tried to turn it over I felt resistance. I took the motor apart and there was a thick black gunk on the 7075 alum. shaft in front and the 7075 alum. bushing in the back. Maybe the 7075 alum. and oil were incompatible or the metals used or both. I cleaned everything up and tried again with the same result.

sb327:
I believe your answer lies more in your successful bushings. Brass is what some describe as ‘self lubricating’ and is why it is commonly used as bushing material.

I do not recall seeing aluminum or ss as a bushing material.

Got a picture of the motor? Sounds neat.

Dave

Motorhead:
If a low rpm model engine & virtually no heat created  ... a Bushing/ Bearing machined from AF grade acetal, MDS nylon, Teflon or even PEEK are IMO the better suited.
None of the above would require addition lubrication ( Which is drag )

KWK:
Is your motor a wobbler design? Oscillating engine?

JPSAXNC:
Thanks for the replies. I thought of using peek for the crankshaft, but I need more weight up front to balance the model so I'll use steel. I don't have any pics yet, but if you google ( Pachasa compressed air engine), It's a twin, mine is a single operating cylinder, built around a crank case. It's supposed to be a sort of scale engine for a Guillows Stearman PT 17 model. On the Pachasa engine the air is fed in from the front side, I wanted my air supply coming in behind  the engine. That's why the crank is in two pieces, the back half of the crank is the valve, air inlet and exaust. It's been a real can of worms.

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